Here are a few java tricks and codes that you can use as a cheat sheet when solving competitive programming problems.
import java.math.BigInteger;
import java.util.*;
public class Document {
public static void main(String args[]) {
Scanner in = new Scanner(System.in);
String str = "Adarsh";
//replace substring in a String
String result = str.replaceFirst("ar", "A");
System.out.println(result);
//String upper case
System.out.println("String upperCase: " + str.toUpperCase());
//String lower case
System.out.println("String lowerCase: " + str.toLowerCase());
//length of string
System.out.println("Length: " + str.length());
//first index in string
System.out.println("First index of 'r': " + str.indexOf('r'));
//last index in string
System.out.println("Last index of 'a': " + str.lastIndexOf('a'));
//String difference
System.out.println("compare with 'Kumar': " + str.compareTo("Kumar"));
//String contains char sequence
System.out.println(str+" contain 'arsh': "+str.contains("arsh"));
//String to char Array
char charArray[] = str.toCharArray();
System.out.print("Char Array: ");
System.out.println(charArray);
//List to Array
List<String> list = Arrays.asList("C", "C++", "Java");
String[] array = list.toArray(new String[list.size()]);
System.out.println(Arrays.toString(array));
//ASCII
System.out.println("ASCII of numbers");
for(int i=48; i<57; i++){
System.out.print((char)i);
}
System.out.println();
System.out.println("ASCII of A-Z");
for(int i=65; i<91; i++){
System.out.print((char)i);
}
System.out.println();
System.out.println("ASCII of a-z");
for(int i=97; i<123; i++){
System.out.print((char)i);
}
System.out.println();
//character Input
System.out.println("Enter a Character");
char ch = in.next().charAt(0);
//Subtring
System.out.println("Substring of "+str+" 0 to 4 is: "+str.substring(1,4));
//String to Integer
String num = "100";
System.out.println("Integer form of '100': "+Integer.parseInt(num));
//Swapping of 2 numbers using XOR:
int a = 4, b=5;
System.out.println("Before Swapping a="+a+", b="+b);
a ^= b;
b ^= a;
a ^= b;
System.out.println("After Swapping a="+a+", b="+b);
//Multiply by 2
System.out.println("Multiply 3 with 2: "+ (3 << 1));
//Divide by 2
System.out.println("Divide 6 with 2: "+ (6 >> 1));
//Number of Digit
System.out.println((int)Math.floor(Math.log10(556)) +1);
//GCD using BigInteger
BigInteger b1 = BigInteger.valueOf(10);
BigInteger b2 = BigInteger.valueOf(5);
System.out.println("GCD of 10 & 5: "+b1.gcd(b2));
//Check Prime Number
System.out.println("Is 9 prime?: "+BigInteger.valueOf(9).isProbablePrime(1));
//Check if number is power of 2 or not
int x = 7;
System.out.println("Is "+x+" power of 2? "+(x!=0 && ((x&(x-1)) == 0)));
//sort array
int numArray[] = {1, 5, 3, 2, 4};
Arrays.sort(numArray);
for(Integer n: numArray){
System.out.print(n + " ");
}
System.out.println();
char ch2 = 'a';
//upper case character
ch2 = Character.toUpperCase(ch2);
System.out.println("Character upper case: "+ch2);
//lower case character
ch2 = Character.toLowerCase(ch2);
System.out.println("Lower upper case: "+ch2);
//List Sorting
List<Integer> numList= new ArrayList<>();
numList.add(5);
numList.add(1);
numList.add(3);
System.out.println("List Before Sorting");
for(Integer n: numList){
System.out.print(n+" ");
}
System.out.println();
Collections.sort(numList);
System.out.println("List After Sorting");
for(Integer n: numList){
System.out.print(n+" ");
}
System.out.println();
//Hash Map
HashMap<String, Integer> numHash = new HashMap<>();
numHash.put("A",65);
numHash.put("B",66);
numHash.put("C",67);
if(numHash.containsKey("A"))
System.out.println(numHash.get("A"));
if(numHash.containsValue(66))
System.out.println(numHash.get("B"));
System.out.println("Size Of HashMap: "+numHash.size());
numHash.remove("A");
System.out.println("Size Of HashMap after removing First Object: "+numHash.size());
numHash.replace("C",0);
System.out.println(numHash.get("C"));
numHash.clear();
if(numHash.isEmpty())
System.out.println("HashMap is Empty");
//Stack
Stack<Integer> stack = new Stack<>();
stack.push(2);
stack.push(3);
stack.push(4);
System.out.println("Top Element in Stack: "+stack.peek());
System.out.println("Popping Top Element in Stack: "+stack.pop());
System.out.println("Top Element in Stack: "+stack.peek());
stack.clear();
if (stack.isEmpty())
System.out.println("Stack is Empty");
}
}Output:
AdAsh String upperCase: ADARSH String lowerCase: adarsh Length: 6 First index of 'r': 3 Last index of 'a': 2 compare with 'Kumar': -10 Adarsh contain 'arsh': true Char Array: Adarsh [C, C++, Java] ASCII of numbers 012345678 ASCII of A-Z ABCDEFGHIJKLMNOPQRSTUVWXYZ ASCII of a-z abcdefghijklmnopqrstuvwxyz Enter a Character p Substring of Adarsh 0 to 4 is: dar Integer form of '100': 100 Before Swapping a=4, b=5 After Swapping a=5, b=4 Multiply 3 with 2: 6 Divide 6 with 2: 3 3 GCD of 10 & 5: 5 Is 9 prime?: false Is 7 power of 2? false 1 2 3 4 5 Character upper case: A Lower upper case: a List Before Sorting 5 1 3 List After Sorting 1 3 5 65 66 Size Of HashMap: 3 Size Of HashMap after removing First Object: 2 0 HashMap is Empty Top Element in Stack: 4 Popping Top Element in Stack: 4 Top Element in Stack: 3 Stack is Empty